26,968 research outputs found
Intercontinental clock synchronization with the block 1 VLBI system
The Block 1 very long baseline interferometer (VLBI) operated by the Deep Space Network (DSN) to make weekly measurements of the relative epoch and rate offsets between the time standards in the global network of DSN stations is discussed. The precision of these measurements routinely achieves sub-microsecond levels for epoch offset and accuracies of better than one part in 10 to the 12th power for rate offset. The implementation of the phase calibrator system permits absolute measurement of epoch offset to better than 10 nanoseconds. With the near-real-time play-back and on-line storage of VLBI data, the Block 1 system typically produces clock parameters within 48 hours from the time of observation
Ellipsoidal Coulomb Crystals in a Linear Radiofrequency Trap
A static quadrupole potential breaks the cylindrical symmetry of the
effective potential of a linear rf trap. For a one-component fluid plasma at
low temperature, the resulting equilibrium charge distribution is predicted to
be an ellipsoid. We have produced laser-cooled Be ellipsoidal ion crystals
and found good agreement between their shapes and the cold fluid prediction. In
two-species mixtures, containing Be and sympathetically cooled ions of
lower mass, a sufficiently strong static quadrupole potential produces a
spatial separation of the species.Comment: 4 pages, 3 figure
Development of a relatchable cover mechanism for a cryogenic IR-sensor
A cover mechanism for use on the Infrared Background Signature Survey (IBSS) cryostat was developed. The IBSS IR-instrument is scheduled for STS launch in early 1991 as a payload of the Shuttle Payload Satellite (SPS) 2. The cover is hinged, with a motorized rope drive. During ground processing, launch, entry, and landing, the cryostat, which houses the IR-instrument, is required to be a sealed vacuum tight container for cooling purposes and contamination prevention. When on orbit, the cover is opened to provide an unobstructed field of view for the IR-instrument. A positive seal is accomplished through the use of latch mechanism. The cover and the latch are driven by a common redundant actuator consisting of dc motors, spur gears, and a differential gear. Hall probe limit switches and position sensors (rotary variable transformer) are used to determine the position of the cover and the latch. The cover mechanism was successfully qualified for thermal vacuum (-25 to 35 C), acoustic noise, vibration (6 Gs sine, 9.7 G RMS) and life cycles. Constricting requirements, mechanical and electronic control design, specific design details, test results of functional performance, and environmental and life tests are described
Stable schedule matching under revealed preference
Baiou and Balinski (Math. Oper. Res., 27 (2002) 485) studied schedule matching where one determines the partnerships that form and how much time they spend together, under the assumption that each agent has a ranking on all potential partners. Here we study schedule matching under more general preferences that extend the substitutable preferences in Roth (Econometrica 52 (1984) 47) by an extension of the revealed preference approach in Alkan (Econom. Theory 19 (2002) 737). We give a generalization of the GaleShapley algorithm and show that some familiar properties of ordinary stable matchings continue to hold. Our main result is that, when preferences satisfy an additional property called size monotonicity, stable matchings are a lattice under the joint preferences of all agents on each side and have other interesting structural properties
Regional scale analysis of the altimetric stream network evolution
International audienceFloods result from the limited carrying capacity of stream channels when compared to the discharge peak value. The transit of flood waves - with the associated erosion and sedimentation processes - often modifies local stream geometry. In some cases this results in a reduction of the stream carrying capacity, and consequently in an enhancement of the flooding risk. A mathematical model for the prediction of potential altimetric stream network evolution due to erosion and sedimentation processes is here formalized. It works at the regional scale, identifying the tendency of river segments to sedimentation, stability, or erosion. The model builds on geomorphologic concepts, and derives its parameters from extensive surveys. As a case study, tendencies of rivers pertaining to the Valle d'Aosta region are analyzed. Some validation is provided both at regional and local scales of analysis. Local validation is performed both through a mathematical model able to simulate the temporal evolution of the stream profile, and through comparison of the prediction with ante and post-event river surveys, where available. Overall results are strongly encouraging. Possible use of the information derived from the model in the context of flood and landslide hazard mitigation is briefly discussed
Quantum phases of atomic boson-fermion mixtures in optical lattices
The zero-temperature phase diagram of a binary mixture of bosonic and
fermionic atoms in an one-dimensional optical lattice is studied in the
framework of the Bose-Fermi-Hubbard model. By exact numerical solution of the
associated eigenvalue problems, ground state observables and the response to an
external phase twist are evaluated. The stiffnesses under phase variations
provide measures for the boson superfluid fraction and the fermionic Drude
weight. Several distinct quantum phases are identified as function of the
strength of the repulsive boson-boson and the boson-fermion interaction.
Besides the bosonic Mott-insulator phase, two other insulating phases are
found, where both the bosonic superfluid fraction and the fermionic Drude
weight vanish simultaneously. One of these double-insulator phases exhibits a
crystalline diagonal long-range order, while the other is characterized by
spatial separation of the two species.Comment: 4 pages, 3 figures, using REVTEX
Synthetic observations of internal gravity waves in the solar atmosphere
We study the properties of internal gravity waves (IGWs) detected in
synthetic observations that are obtained from realistic numerical simulation of
the solar atmosphere. We used four different simulations of the solar
magneto-convection performed using the CO5BOLD code. A magnetic-field-free
model and three magnetic models were simulated. The latter three models start
with an initial vertical, homogeneous field of 10, 50, and 100 G magnetic flux
density, representing different regions of the quiet solar surface. We used the
NICOLE code to compute synthetic spectral maps from all the simulated models
for the two magnetically insensitive neutral iron lines Fe I 5434 {\AA} and
5576 {\AA}. We find the signatures of the internal gravity waves in the
synthetic spectra to be consistent with observations of the real Sun. The phase
differences obtained using the spectral lines are significantly different from
the phase differences in the simulation. The phase coherency between two
atmospheric layers in the gravity wave regime is height dependent and is seen
to decrease with the travel distance between the observed layers. We conclude
that the energy flux of IGWs determined from the phase difference analysis may
be overestimated by an order of magnitude. Spectral lines that are weak and
less temperature sensitive may be better suited to detecting internal waves and
accurately determining their energy flux in the solar atmosphere.Comment: To appear in Astronomy & Astrophysics, 12 pages, 12 figures, abridged
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Spin-orbital excitation continuum and anomalous electron-phonon interaction in the Mott insulator LaTiO
Raman scattering experiments on stoichiometric, Mott-insulating LaTiO
over a wide range of excitation energies reveal a broad electronic continuum
which is featureless in the paramagnetic state, but develops a gap of cm upon cooling below the N\'eel temperature K. In the
antiferromagnetic state, the spectral weight below the gap is transferred to
well-defined spectral features due to spin and orbital excitations. Low-energy
phonons exhibit pronounced Fano anomalies indicative of strong interaction with
the electron system for , but become sharp and symmetric for . The electronic continuum and the marked renormalization of the phonon
lifetime by the onset of magnetic order are highly unusual for Mott insulators
and indicate liquid-like correlations between spins and orbitals.Comment: to appear in Phys. Rev. Let
Itinerant-electron Ferromagnetism in W(Nb)O3-d
The crystal structure and the magnetic properties of the W1-xNbxO3-d,
(x<0.03) system have been investigated. In contrast to the orthorhombic
diamagnetic WO3, the material with x=0.01 is paramagnetic down to 5 K.
Introducing of 2.5 at. % of Nb into WO3 leads to a tetragonal structure and to
a weak itinerant ferromagnetic ordering below TC= 225 K. The saturation
magnetic moment at 5 K is 1.07*10-3 mB, whereas the paramagnetic effective
moment is 0.06 mB per mole. This high ratio indicates itinerant ferromagnetism
in W0.975Nb0.025O3-d.Comment: accepted to Physica
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